1. Field Of The Invention
This invention relates to protective devices for positive displacement pumps, and more particularly, to a protective cover assembly having a reverse buckling disc which buckles rather than shears when subjected to excessive pressure.
2. Description Of The Prior Art
It is common practice in the petroleum industry to employ high pressure plunger-type pumps in a variety of field operations relating to oil and gas wells, such as cementing, acidizing, fracturing and others. An example of such a high pressure pump is the Halliburton Services HT-400 Horizontal Triplex Pump manufactured by Halliburton Services of Duncan, Okla. Such pumps commonly generate pressures in excess of 10,000 psi, and are on occasion subject to overpressuring for a variety of reasons. Several common causes of overpressure are blockage of a pump discharge line, the erroneous closure of a valve on the discharge side of the pump, or the phenomenon of "sandout".
Sandout may occur during a fracturing job, wherein the producing formation of the well is subjected to high pressures to "fracture" the producing strata. It is common in such fracturing operations to include a proppant, such as glass or ceramic beads, walnut shells, glass microspheres, sintered bauxite, or sand (hereinafter collectively and individually referred to as "sand") in the carrier fluid, so as to provide a means of maintaining the cracks in the fracturing producing formation open after the fracturing pressure is released. Present day fracturing operations often employ a foamed carrier fluid such as nitrogen or carbon dioxide as the gaseous phase of the foam, in order to lower the volume and cost of the chemicals required and in many cases to avoid a large hydrostatic force on a well formation, such as is often encountered in gas wells.
There has also recently been a marked tendency to load up the carrier liquid with as much sand as possible prior to foaming, in order to further lower fluid volume requirements and hence job costs to the customer. Such concentrations may reach and exceed sixteen pounds of sand per gallon of carrier fluid. These high sand concentrations impose severe performance demands on the blender, manifold and pump systems due to the erosive effect of the sand and the tendency of slugs of sand to collect in valves, elbows, and in the fluid ends of the high pressure pumps. A collection of sand in these areas is dependent upon a number of parameters, including gravity, fluid flow rate, rheological properties of the carrier fluid, physical properties of the sand, and the geometry of the system as a whole.
However, regardless of causation, the concentration of sand associated with the sandout in the fluid end of a high pressure pump can result in sudden overpressuring of the fluid end with resulting damage to one or more of the plunger, connecting rod, crankshaft, fluid end or other parts of the pump drive train. The overpressuring due to sandout is particularly destructive as the resulting force may be eccentrically applied to the plunger and fluid end, as a slug of sand often collects at the bottom of the plunger.
It has been well known in the art to attempt to alleviate this sandout problem with ball-type valves in the pumps. However, such valves are susceptible to clogging due to the sand content of the carrier liquid, and may also fail to reclose after the problem is corrected due to the presence of sand in the valve or the erosive effect of the sand laden carrier fluid.
Another solution to the overpressuring problem is disclosd in U.S. Pat. No. 4,508,133 to Hamid, assigned to the assignee of the present invention. This invention comprises a protective cover assembly including a substantially circular cover having a shear disc surrounded by an annular outer portion, mounted in a cylinder in the fluid end of the plunger-type high pressure pump. An arcuate boundary of reduced wall thickness lies between the shear disc and the outer portion of the cover. The cover is held in place by a retainer assembly which is secured to the fluid end, which retainer assembly includes a plug backed by an impact disc at the outer end of the retainer. When a predetermined force is generated by the plunger and the cylinder, the shear disc of the cover shears and is propelled outwardly against the plug, which in turn forces the impact disc against the edge of a circular recess in the outer end of the retainer, the recess being of lesser diameter than the impact disc. The impact disc, in shearing against the recess edge, safely dissipates the kinetic energy of the shear disc, while the pressure in the cylinder vents to the atmosphere, avoiding damage to the fluid end of the pump, the plunger, connecting rod, crankshaft, etc., as well as potential damage to the well head. However, the retainer employed with a protective cover is expensive to construct, and in order to refurbish a sheared cover and retainer assembly, a new impact disc as well as a new cover must be available. Moreover, the use of a destructible impact disc to absorb energy adds to the operating costs of the pump in which they are employed.
U.S. Pat. No. 4,520,837 to Cole et al., also assigned to the assignee of the present invention, discloses a protective cover with a shear disc essentially the same as in Hamid, but also includes a more simple, one-piece cover retainer inserted behind the protective cover. When the shear disc is subjected to a load in excess of the shear strength of the arcuate boundary thereon, the cover shears along the boundary and the shear disc is propelled outwardly by the pressure in the fluid end into the cover retainer, the interior of which is of substantially frustoconical configuration, with the base of the cone oriented substantially coaxially with respect to the shear disc. The kinetic energy of the shear disc is substantially dissipated by the contact of the periphery of the disc with the ever decreasing diameter inner wall of the retainer, which plastically deforms the shear disc. The fluid end of the pump, the plunger, connecting rod, crankshaft, etc., are saved from harm by the venting of the overpressure when the disc shears. After the cover retainer with the trapped shear disc and the sheared cover outer portion are removed from the fluid end of the pump, the sand is cleared from the fluid end (if sandout is the cause of the overpressure), a new protective cover is installed, the cover retainer resecured to the fluid end, the pump restarted and the fracturing operation recommenced.
While the apparatus of Cole et al. has advantages over the apparatus of Hamid, there are still a number of problems remaining. First of all, the shear disc is subjected to cyclic loading. This cyclic stress causes fatigue and premature failure of the disc around the thin arcuate wall may occur even at low pump pressures. Another problem is that the thin area around the arcuate portion does not leave much thickness for corrosion allowance, and thus may fail prematurely when corrosion is present. A further problem with the previous apparatus is that the shear disc is expensive to fabricate, and machining will invariably leave machine marks which act as stress risers and compound the fatigue problem already mentioned.
The present invention solves these problems by providing a protective cover assembly with a reverse buckling disc which has a convex surface exposed to the pressure in the pump and thus is loaded in compression only. This greatly improves fatigue life. The convex shape of the disc is easily stamped, thereby eliminating machining marks and the problems related therewith. Because there is no thin section, corrosion is not a great problem.
Another advantage of the present invention is that, under normal circumstances, no fluid is vented out of the pump because of the buckling action of the reverse buckling disc.